This invention relates to telephone line circuits. More particularly it relates to telephone line circuits for providing a holding function and means for indicating that the holding function has been engaged for a predetermined time.
Telephone line circuits have been provided for switching lamp current at different flash rates to indicate an incoming call, an answered call, or a call that has been placed on hold. These circuits further provide a means for initiating a local ringer as well as providing a holding impedance which simulates the impedance of the telephone receiver when a call has been placed on hold. Most of these switching functions in the past have been performed by coil and contact relays. The contacts in this type of relay have a tendency to malfunction when relatively high currents are switched and when a large number of switching functions have been performed. The relay which have been most troublesome have been the relay supplying lamp current since the contacts must carry relatively high current to light the station instrument lamps.
One of the functional problems in the use of a key telephone system is the annoyance to an incoming caller who has been placed on hold for a substantial length of time. The attendant quite often forgets that the incoming call has been placed on hold. Therefore, it is desirable that the attendant be periodically reminded that a call has been left on hold. This would enable the attendant to tell the party left on hold the status of his call.